Technical Field
The present invention relates to a zoom lens, and particularly to a projection zoom lens to be applied to a projection type display device.
Further, the present invention relates to a projection type display device equipped with such a projection zoom lens.
Description of the Related Art
Conventionally, projection type display devices that utilize light valves, such as liquid crystal display elements, DMD's (Digital Micromirror Device: registered trademark), or the like have been widespread. Further, in recent years, movie theaters and the like have been using such projection type display devices, which are designed to be capable of being applied to large screens and of producing high-definition images.
Such projection type display devices to be utilized in movie theaters and the like as described above have been required to have long back focus and a favorable telecentricity. This is because such projection type display devices adopt a three-plate system in which three light valves for the respective primary colors are provided. In the three-plate system, a light beam from the light source is divided into three primary colors by a color separation optical system, and the separated light of the three colors passes through the respective light valves and then is composed by a color composition optical system to be projected.
Further, such projection type display devices to be utilized in movie theaters and the like have been required to be equipped with high zoom-ratio zoom lenses as a projection lens. This aims at fitting the size of display images to a screen size in accordance with the projector distances and screen sizes, which differ depending on movie theaters, halls, and in accordance with aspect ratios (cinema scope, Vista size) of display images.
Further, there are many cases that such projection zoom lenses are required to maintain constant numerical apertures (hereinafter, referred to as “F number(s)” as well) over the entire zoom range. This is because operating in such a manner can prevent the brightness of display images from changing when the zoom ratios of this type of zoom lens are changed to match the sizes described above.
Further, accompanying the rapid pace of digitalization of cinema screens, projection type display devices have been rapidly miniaturized and reduced in cost. There is likely to be also demand for projection zoom lenses to be miniaturized and reduced in cost in addition to the demands for back focus, telecentricity, and high zoom ratio.
Patent Documents 1 and 2 (Japanese Unexamined Patent Publication No. 2009-128683 and Japanese Unexamined Patent Publication No. 2012-058607) disclose projection zoom lenses which are intended for use in projection type display devices. More specifically, Patent Documents 1 and 2 disclose the projection zoom lens in which a first lens group having a positive refractive power positioned at the most-magnification side and a final lens group having a positive refractive power positioned at the most-reduction side are fixed while changing magnification, and a moving lens group as an intermediate group positioned between these lens groups is caused to move while changing magnification. In such projection zoom lenses, the numerical apertures are maintained constant over the entire zoom range.
Patent Documents 1 and 2 discloses projection zoom lenses that consist of a second lens group having a negative refractive power (hereinafter, referred to simply as a “negative” lens), a negative third lens group, and a positive fourth lens group as an example of the intermediate group mentioned above. Patent Document 2 also discloses a projection zoom lens in which an intermediate group consists of a negative second lens group and a positive third lens group. Patent Document 2 further discloses a projection zoom lens in which the intermediate group consists of a negative second lens group, a positive third lens group, and a positive fourth lens group and which fails to have the function of maintaining the numerical aperture constant.